1. Field of the Invention
The present invention relates to computer data storage devices and, in particular, relates to a method of erasing a disk during a manufacturing process.
2. Description of the Related Art
Hard disk drive storage devices are an important component in virtually all computer systems. In particular, hard disk drives provide computer systems with the ability to store and retrieve data in a non-volatile manner such that the data is maintained even if power is removed from the device. The popularity of these devices is based on their ability to quickly store and retrieve large quantities of digital information at low cost.
The typical hard disk drive comprises one or more pivotally mounted disks having a magnetic recording layer disposed thereon and a plurality of magnetic transducer elements for affecting and sensing the magnetization states of the recording layer. The recording layer comprises a large number of relatively small domains disposed thereon that can be independently magnetized according to a localized applied magnetic field and that can be maintained in the magnetized state when the external field is removed. The domains are generally grouped into concentric circular data tracks each having a unique radius on the disk and data is written to or read from each track by positioning a transducer adjacent the disk at the corresponding radius while the disk is rotated at a fixed angular speed.
The disk further comprises a plurality of servo wedges defining servo tracks disposed adjacent the data tracks. The servo wedges function as reference markers to permit a controller to determine the location of the transducer on the disk in an accurate manner. The servo wedges are written to the magnetic media of the disk during a manufacturing process called the servo track write process. This process typically occurs after the disk drive is in a near completed state. In particular, the disk(s) is mounted on a spindle to permit rotation, and the transducer is mounted on an actuator to permit movement relative to the surface of the disk. As is well known in the art, the motion of the actuator (and thus the transducer) is typically performed by a voice coil motor (VCM) that has a permanent magnet interacting with an electromagnet.
As with any manufacturing process, some disk units end up with faulty written information, such as the servo wedges, on the magnetic media Faulty servo wedges can lead to the controller being unable to accurately position and maintain the transducer on the tracks defined by the faulty servo wedges. As a result, data capacity and overall performance of the disk is reduced. To correct this problem, these disks may undergo complete magnetic erasure to yield a ‘clean slate’ of magnetic media on which new servo wedges can be rewritten. One method of erasing the disk is to subject the entire disk to a magnetic field with a predetermined field strength to demagnetize the existing magnetized domains such as the servo wedges. A drawback to such a complete demagnetization is that other parts of the disk drive also are subjected to the magnetic field and may become adversely affected. As an example, permanent magnet of spindle motor can become partially demagnetized, thus reducing the spindle motor's startup torque generation capability. Other parts in the disk drive, such as the VCM can also become partially demagnetized by the magnetic field.
Another method of erasing the disk comprises a partial erasure of a portion of the disk using the external magnetic field, followed by a track-by-track erasure using the transducer. In this method, the hard disk drive is partially inserted between two magnet pole pieces, between which a magnetic field exists. The strength of the magnetic field is selected to erase the magnetic media of the disks. A structural member of the hard disk drive is attached to a carriage that controls how far the hard disk drive moves into the magnetic field. The hard disk is preferably positioned so that the VCM end of the actuator and the spindle are not subjected to the full strength of the magnetic field.
When a portion of the hard disk is placed in the magnetic field, the rotation of the disks causes the outer portion of the disks to be erased, while leaving the inner portion unerased. The inner portion of the disk is subsequently erased using the transducer, whereby the structural member of the hard disk drive is mounted to a servo track writer carriage so as to provide an accurate positioning of a push rod relative to the actuator and the disk. The disk is rotated while an erase signal is applied to the transducer such that one rotation of the disk with the stationary transducer yields an erased band that is substantially circular.
The erasing process preferably begins near the boundary between the mass-erased outer portion and the unerased inner portion, such that transducer erased bands overlap a portion of the outer portion to ensure substantially complete erasure. Once the first circular erased band is made, the transducer is moved inward by a push rod pushing on the actuator in a predetermined manner so as to erase a smaller radius circular path.
To ensure that substantially all of the inner portion is erased, a given erased band overlaps with the previous erased band and the next erased band. To accommodate such overlaps, the transducer is typically moved inward in half-servo track width increments. The width of the erased band is typically less than the Width of the servo track. Thus, shifting of the transducer in half-servo track width increments yields areas of erasure overlap between the two adjacent erased bands. Therefore, it typically takes two erased bands to erase one servo track.
In addition to the inefficient overlapping of the erased bands, the current method of erasing the disk in overlapping concentric circular tracks suffers from time consumed during repositioning of the transducer to a new circular path. As is well known in the art, positioning of the transducer involves settling of the transducer on a target track, a process that typically takes time in the order of few milliseconds. Since a typical disk comprises number of servo tracks in the order of tens of thousands, few milliseconds per track adds up to a significant amount of time.
To appreciate how track-by-track erasing process is relatively time consuming, consider an exemplary disk with 45,000 tracks. Suppose that half of the disk is already mass erased by an external magnet, leaving 22,500 tracks to be erased by the transducer. Since two rotations (two half-track width increments) erases one track, the transducer needs to be positioned 2×22,500 or 45,000 times. If the settling time is 7 ms, then the total time taken up by settlings alone is 315 seconds, or approximately 5 minutes. It will be appreciated that this time is in addition to the total time taken during erasing with the transducer. It will also be appreciated that erasing track by track (and thus being bound with settling times) is an inefficient method, considering that the end result is a simple erased disk surface.
In a mass production, the extra time per unit to settle the transducer on the tracks, as well as the time required to have overlapping erase paths, translates to a significant cost, especially if the limited number of servo track writing apparatuses forms a bottleneck in the overall production process. Thus, there is a need for a method that reduces the overall time used to erase a disk using a transducer. To this end, there is a need for a method of erasing at least a portion of the disk surface that does not require as much overlap of the erase path and also reduces the amount of time needed to settle the transducer on each of the tracks to be erased.